1. Field of the Invention
The present invention relates generally to an ink-jet printing method and an apparatus, and particularly, to an ink-jet printing method and an apparatus in which a liquid making a coloring agent of an ink insoluble or coagulated, is applied to the ink in an overlapping manner.
2. Description of the Related Art
Conventionally, an ink-jet printing method has been widely used in printer, copy machine and so on, for a low noise, a low running cost, an easiness of down-sizing of an apparatus, an easiness of color printing and other various advantages.
In the case of printing of an image on a printing medium, particularly on the printing medium called as a plain paper by apparatuses employing the ink-jet printing method, it is possible that water resistance of the image on the printing medium may have been insufficient. Also, upon printing of a color image, especially of a high density color image, difficulty may have been encountered in achieving both of suppression of feathering and preventing from bleeding between different colors. Therefore, it has been relatively difficult to obtain a high quality color image having high image fastness.
As a method for improving the water resistance of the image printed on the printing medium, inks provided water resistance for a coloring agent contained therein have been practiced in the recent years. However, even with such ink, water resistance of the ink has not yet been sufficient. In addition, as a nature, such type of the ink is difficult to solve in water after once dried to possibly cause plugging in ejection openings and the like of a printing head. Furthermore, in order to prevent plugging in the ejection openings and the like, a construction of the apparatus becomes complicate.
It has also been known a large number of technologies in improving fastness of printed products. For example, in Japanese Patent Application Laid-open No. 24486/1978, a technology for fixing a dye by laking through a post-processing of dyed product in order to increase wet color fastness of the dyed product.
Also, in Japanese Patent Application Laid-open No. 43733/1979, there has been disclosed a method for performing printing by using two or more components which increase layer forming ability under normal temperature or heating when the components are mutually contacted, by means of the ink-jet printing system. By the disclosed method, a printing product with firmly fixed layer has been obtained by contacting the respective components on a printing medium.
Furthermore, in Japanese Patent Application Laid-open No. 150396/1980, there has been disclosed a method for applying a waterproofing agent which forms a color lake with the dye after printing by a water-base dye ink.
Also, in Japanese Patent Application Laid-open No. 128862/1983, there has been disclosed an ink-jet printing method, in which an image position to be printed is preliminarily identified and a printing ink and a processing ink are applied in an overlapping manner. In this publication, there has been disclosed methods printing the image with the processing liquid in advance of printing with the printing ink, printing with the processing ink overlapping on the image preliminarily printed by the printing ink, or overlappingly printing with the printing ink over the image preliminarily printed by the processing ink and further printing with the processing ink thereover.
Furthermore, in Japanese Patent Application Laid-open No. 52867/1996 of the assignee of the present application, there has been disclosed a method of applying a processing liquid which makes a coloring agent of the ink insoluble or coagulated, over each pixel at a predetermined ratio.
In addition, in Japanese Patent Application Laid-open No. 226154/1997 of the assignee of the present application, attention has been paid for an edge portion of the image to be printed to eject the processing liquid at the predetermined ratio for the portion other than the edge portion of the image, and on the other hand, to eject the processing liquid over the edge portion of the image entirely so as to certainly attain the water resistance with avoiding unnecessarily wasting the processing liquid.
It should be noted that the foregoing processing liquid is effective not only for improving to water resistance of the printed image but also for increasing density, avoiding bleeding and so on. In such viewpoint, the processing liquid will also be referred to as a printing ability improving liquid. Namely, throughout the present specification, the wording "processing liquid" and the wording "printing ability improving liquid" are used in the same meaning.
In the ink-jet printing apparatus, it has been known that a problem is encountered in degrading the image quality caused due to density unevenness.
One of primary causes of the density unevenness is slight errors in heater portion, shape of the ejection openings and so on in the printing head caused during manufacturing process thereof. Such error should cause fluctuation of ink ejection amounts and/or of ejecting direction of respective ejection openings during printing to result in the density unevenness on the printed image.
FIGS. 9A to 9C show one example of the density unevenness caused in the printed image. FIG. 9A is a diagrammatic illustration showing the printing head constituted of eight ink ejection openings and also showing fluctuation caused in volume and direction of the ink ejected through respective ink ejection openings. When printing is performed using such printing head, dots different in a size and a position for respective rows respectively corresponding to the respective ink ejection openings are formed, as shown in FIG. 9B. As a result of this, the density unevenness so-called white stripe, in which non-printed blank portion is cyclically expressed in relatively strong or so-called black stripe, in which adjacent dots overlap excessively, and so on may be caused. FIG. 9C shows a density distribution of such dots formed as described above.
On the other hand, as a system for solving the density unevenness due to fluctuation of ejection characteristics for each individual ejection opening as set forth above, it has been well known that a multi-path system (or multi-scanning system) is effective. This system is a system for printing pixels in each line in a primary scanning direction by a plurality times of scan and for forming dots in each line with ink ejected through a plurality of mutually different ink ejection openings, or a system for printing each of pixels in each line by a plurality of times of scan to form each pixel with inks ejected from a plurality of mutually different ink ejection openings. The later system is the multi-path system to be employed in a multi-tone printing or density enhancing printing for printing each pixel with a plurality of ink droplets.
FIGS. 10A to 10C are explanatory illustrations in the case where the former multi-path system is implemented with the same printing head as that of FIG. 9A. As shown in FIG. 10A, the eight ejection openings of the printing head is divided into two groups respectively consisted of upper four ejection openings and lower four ejection openings. Also, dots to be formed through one scan by respective ejection openings in respective group correspond a thinned image data which is thinned into substantially half from one line of an original image data by a predetermined method. After printing for one scan, paper feeding for four pixels is performed to oppose ejection openings different from those use in the preceding scan (in different group), to the line to be printed for forming dots on a basis of the remaining half of the image data to finally complete printing of the line. Thus, each of raster (one line in the scanning direction) can be printed with inks ejected from different ejection openings. Therefore, influence of fluctuation of the ejection characteristics for each ejection opening can be reduced and whereby to reduce density unevenness in the printed image, as shown in FIGS. 10B and 10C.
Various dividing method of the image data in the multi-path system identifying dots in each line to be printed in each scan have been disclosed, conventionally. In addition to the dividing method employing a fixed mask thinning data per each dot (each pixel) as set forth with reference to FIGS. 9A to 9C, there has been known a sequential multi-scanning system (hereinafter referred to as SMS), in which ejection openings to be used for respective lines are varied cyclically, as disclosed in Japanese Patent Application Laid-open No. 330083/1993. A method for performing enhancing or thinning printing employing the foregoing SMS has been proposed in Japanese Patent Laid-open No. 157113/1998 of the assignee of the present application.
However, the inventors of the present application have found the following new problems to be encountered in application of the foregoing processing liquid in the foregoing multi-path system.
In general, the ink and the processing liquid may extend beyond the pixel to be printed when they are applied to a printing medium, such as a printing paper or the like. Particularly, when the ink or the processing liquid having high permeability is used or when the ejection amount per one pixel is sufficiently large, a diameter of the dots formed on the printing medium may become greater for spreading of the ink or the like. As a result of this, in certain case, overall surface of the printing medium may be covered with the ink or the processing liquid without requiring application of the ink or the processing liquid for all of the pixels. FIGS. 11A to 11C show the case where overall surface of the printing medium can be covered with the ink and the processing liquid without applying the ink or the processing liquid for all of the pixels.
FIG. 11A is a diagrammatic illustration of the image data in the case where an ejection duty is set at 50% in an image to be printed at a pixel pitch of 600 dpi. Namely, each pixel is expressed as a region defined by a grid of 42 .mu.m. By applying the ink or the processing liquid in the pixel shown with hatching to establish 50% ejection duty.
FIG. 11B is an illustration showing an example where the dot diameter on the surface of the printing medium is large for high permeability of the ink or the processing liquid, or for large ejection amount. In the shown example, the dot diameter is 80 .mu.m. As shown in FIG. 11B, while the ejection duty of the data for applying the ink is 50%, the ink or the processing liquid may cover the entire surface of the printing medium with enlarging of the dot diameter due to spreading of the ink or the like on the surface of the printing medium.
In contrast to this, when the dot diameter is small because the permeability of the ink or the processing liquid is relatively low or because the ejection amount is small, a region not covered by the ink or the processing liquid may be formed on the surface of the printing medium, as shown in FIG. 11C. In such case, unless the ink or the processing liquid is applied with the ejection duty of 100%, the entire surface of the printing medium cannot be covered. In the example shown in FIG. 11C, the dot diameter is about 50 .mu.m.
As shown in FIG. 11B, when the entire surface of the printing medium is covered with the processing liquid without no blank portion, the processing liquid may achieve effect for improving the printing ability. In this case, by generating the data for applying the processing liquid corresponding to the printing data (data for ejecting the ink), the processing liquid can be effectively applied to the pixels, to which the ink is applied. In such case, when thinning the processing liquid ejection data in taking account of spreading of the processing liquid and applying a necessary minimum amount of the processing liquid, application amount of the processing liquid can be restricted to be small. Also, reducing the application amount of the processing liquid is also effective for suppressing occurrence of cockling due to wetting of the printing paper. Furthermore, reduction of application amount of the processing liquid may contribute for lowering of a running cost.
However, when printing in the multi-path system is to be performed, it is possible that the application pattern of the processing liquid becomes synchronous with the mask of the multi-path in certain thinning method of the processing liquid to negate the effect of the multi-path system. One example of such case will be explained with reference to FIGS. 12A to 12G.
FIG. 12A shows an image data for printing an image consisted of four pixels in the longitudinal direction and two pixels in the lateral direction. When the processing liquid is applied in a pattern thinned into 50% as shown in FIG. 12B for this printing data, the dots formed with the ink and the processing liquid and the dots formed only with the ink are arranged in a checkered pattern, as shown in FIG. 12C. In this case, in order to effectively improve the printing ability, the system is constructed to print the processing liquid and the ink are printed in the same scan so that the processing liquid is applied immediately before printing by the ink.
For simplification, it is assumed that the image shown in FIG. 12C is printed by the printing head having respective four ink ejection openings for respective of the ink and the processing liquid as shown in FIG. 12D. In this printing head, the ejection opening groups for respective of the ink and the processing liquid are arranged along the scanning direction so that the processing liquid is applied to each pixel in advance of application of the ink.
On the other hand, a division mask used for the multi-path printing is a fixed mask in the checkered pattern for the thinning method for completing the image by two path (two scans) shown in FIG. 12E. More specifically, as shown in FIG. 12E, by printing the pixels identified by respective hatching in the first scan and the second scan, data is mutually complemented by scanning twice.
FIG. 12F illustrates a method for printing the image of FIG. 12A employing the thinning mask of FIG. 12E by the printing head of FIG. 12B. At first, by scanning (first scan) of the printing head at the first time, diagonally positioned two dots of pixels are formed. These two pixels are pixels, to which the processing liquid is applied with the mask for the processing liquid (FIG. 12B). By this, on these two pixels, the dot in which the processing liquid and the ink are overlaid is formed. Next, after paper feeding (the drawing is illustrated as if the head is moved) for two ejection openings, a second scan is performed. In the second scan, the mask of the second scan shown in FIG. 12E is used. However, the pixels to be printed at this second scan are the pixels, to which the processing liquid is not applied in accordance with the mask for the processing liquid shown in FIG. 12B. More specifically, the mask for the processing liquid (FIG. 12B) and the mask for ink ejection (FIG. 12E) are synchronized. As a result of this, in the second scan, the processing liquid is not applied at all and only the ink is applied. In the final, third scan printing, the mask of the first scan shown in FIG. 12E is used to overlay the processing liquid with the ink.
In FIG. 12F, while an arrangement of dots is diagrammatically illustrated, the actually printed dots have greater dot diameter to be possibly applied to a periphery of the pixel beyond the boundary of the pixel (FIG. 12G). In this case, in FIG. 12F, a region can be divided into two regions A and B depending upon an order of sequent in applying the processing liquid. In a region (region A) where the processing liquid is applied in the first scan, the processing liquid is widely spread in the first scan as shown in FIG. 12G. It is equivalent to the case where the processing liquid is applied even for the pixel to be not printed. In the region A of FIG. 12F, only ink is applied to the non-printing pixel at the first scan in the second scan. From the foregoing, the processing liquid has been already spread even in the pixel to be printed only by the ink. Therefore, it becomes equivalent to print all of the pixels of the region A in the sequential order of the processing liquid and then the ink.
On the other hand, in the region (the region B) in which, among the twice scan for the region, only ink is applied in the first scan, and both the processing liquid and the ink is applied in the next scan, to the pixel printed in the first scan, the ink is applied at first, and next, the processing liquid spreaded from adjacent pixels is overlaid on the ink. More specifically, in the pixel printed in the first scan, the ink and the processing liquid are overlaid in the sequential order of the ink and then the processing liquid. In contrast to this, the pixel to be printed in the later scan, the processing liquid is applied, at first, and subsequently the ink is applied.
As set forth above, to the region A, the ink is applied after application of the processing liquid over substantially all region. In contrast to this, in the region B, to half in number of pixels, the ink and the processing liquid are applied in the sequential order that the processing liquid is applied at first and then the ink is applied. In the remaining half in number of pixels, the ink and the processing liquid are applied in sequential order that the ink is applied at first, and then the processing liquid is applied.
The printing color of the ink may be varied in case of use together with the processing liquid, and the printing color may also be varied depending upon the sequential order of application of the processing liquid and the ink. Therefore, the printing colors in the region A and B can be different. As a result, in the paper feeding direction of the printing image, the printing region corresponding to the region A and the printing region corresponding to the region B appears alternately to cause stripe form color fluctuation or density unevenness.
As set forth above, when the processing liquid is applied in accordance with thinned data, degradation of image quality can be caused by the color irregularity or the density unevenness due to difference of sequential order in application of the processing liquid at respective regions depending upon the order to scan in the multi-path printing.
Since the processing liquid is unevenly ejected for each scan, reduction of influence of fluctuation of the ejection characteristics for each ejection opening becomes not effective. Also, an amount that the ink and the processing liquid are ejected simultaneously can be increased to cause an adverse effect.